MAN: Explain how does auto_private_groups affect subdomains
    
    Fix explains how auto_private_groups affects subdomains.
    a. POSIX sudomains, gets inherited to subdomain.
    b. ID-mapping subdomains, already enabled.
    
    Resolves: https://pagure.io/SSSD/sssd/issue/3627
    
    Reviewed-by: Rob Crittenden <rcritten@redhat.com>